Heat transfer systems, such as air conditioning systems and heat pump systems, are well known in the prior art. In such systems a working fluid which can be any one of a number of refrigerant materials is used to transfer heat from one region to another.
The working fluid typically passes through a system that includes an evaporator, a compressor, a condenser and an expansion device. Of course the system may also include other components such as an accumulator or a receiver/dryer.
In an air conditioning application the evaporator is positioned in the space to be cooled and the condenser is positioned in the area to which heat removed from the cooled space is transferred. Working fluid in the vapor state is pumped by the compressor into the condenser. In the condenser the working fluid rejects heat and condenses to a liquid.
The liquid working fluid passes from the outlet of the condenser into an expansion device. Expansion devices known in the prior art include fixed orifices, capillary tubes and expansion valves.
From the expansion device the working fluid flows to the evaporator wherein it absorbs heat and undergoes a change of phase from liquid to vapor. The refrigerant vapor then flows back to the compressor to begin another pass through the system.
The expansion device is an important element of the system. The amount of working fluid that passes through the expansion device is a controlling factor in the amount of cooling that can be achieved. However, because the temperature of both the space being cooled and the space to which heat transferred vary, the pressure and temperature of the liquid working fluid entering the expansion device also varies. This impacts the cooling capabilities of the system and affects the flow rate that must be obtained through the expansion device to achieve the optimum cooling effect. In mobile systems such as those used as air conditioning or refrigeration systems on vehicles, the properties of the working fluid delivered to the expansion device can vary widely.
Due to the variable operating conditions of vehicle heat transfer systems, fixed opening expansion devices such as orifices and capillary tubes are sometimes used to reduce cost but are not preferred. Instead, expansion valves that provide variable refrigerant flow rates are more desirable.
Prior art expansion valves have traditionally controlled flow by passing the fluid through an internal opening in the valve and employing a movable restricting body or other element in close proximity to the opening. Moving the restricting body closer to the opening reduces flow. Conversely, moving the restricting body away from the opening increases flow through the valve.
In prior art expansion valves the position of the restricting body has been controlled by an actuator. A common actuator is a diaphragm type which is mounted on the valve. The actuator opens or closes flow through the expansion valve in response to fluid pressure both inside the valve and from a control source.
The control source fluid pressure for moving the restricting body is delivered from a sealed bulb which holds a carefully determined fluid charge. The bulb is commonly mounted adjacent to the outlet line from the evaporator. When the temperature of the working fluid exiting the space to be cooled begins to increase, the temperature of the bulb also increases. As the pressure of the fluid inside the bulb increases it moves the diaphragm and the blocking body inside the expansion valve to increase the flow rate of working fluid to the evaporator. The increased flow of working fluid provides more cooling and eventually the temperature at the outlet of the evaporator drops. When this occurs the pressure inside the bulb falls, moving the diaphragm and the restricting body to reduce the flow rate of working fluid through the valve. The charge in the bulb is contrived to have a small amount of superheat in the refrigerant leaving the evaporator.
A problem with this type of prior art actuator is that the expansion device is constantly seeking the optimum rate of flow. The response time renders the expansion device unable to react properly to changing conditions. This is particularly a problem in vehicle applications where changes in cooling loads and variations in refrigerant properties are common. As a result, the accuracy of control is also less than optimum.
Others have previously used electrically controlled expansion valves to control the flow of working fluid in a heat transfer system. These systems typically use valves that are either fully open or fully closed. The valve is periodically opened and closed for controlled time periods to achieve an overall average flow rate that is designed to handle the heat transfer load at the evaporator.
A significant problem with such pulse width modulated expansion valves is that they must open and close very frequently. This causes rapid wear of the valve components. The opening and closing action also often causes "hammering" in the system. The vibration associated with hammering may cause fatigue and premature failure of the valve and the connected tubing. It also makes accurate pressure measurement impossible.
The control elements used in prior art expansion valves for controlling or regulating the flow of working fluid also have drawbacks. The valves that meter flow must be made to deal with the static and dynamic pressure effects created by the working fluid as it passes through the valve. In some designs efforts are made to use the pressure of the working fluid to develop balancing forces. These balancing forces enable more precise movement of the restricting body or other control element. This is intended to enable more precise control of the flow rate.
The problem with attempts to design expansion valves that make use of such balancing forces is that the forces vary substantially with the fluid conditions and the flow rate. As a result it has been difficult to produce an expansion valve that provides accurate control of fluid flow over a wide range of operating conditions.
Thus there exists a need for an expansion valve for heat transfer systems that provides accurate flow control for the working fluid under a wide range of operating and flow conditions.